In the fabrication of a ceramic package containing, for example, a semiconductor device, the parts are assembled as shown in the drawing. It is to be understood that the package pins 18 are arrayed in plurality along the edge of the finished package and function to connect the semiconductor device to parts (not shown) that are external to the package. In the package a ceramic base 12 has a metal leadframe 18 attached thereto by means of a glass layer. Then the semiconductor device 14 is mounted upon the base and connections 13 are made between the semiconductor device 14 bonding pads and the leadframe pins 18. Then a ceramic cap 10 having a glass coating is pressed against the upper face of the exposed base. The assembly is heated to above the glass fusion temperature whereby the glass layers on the cap and base fuse together to form a glass seal 16. This forms a sealed unitary structure in which the ceramic cap 10 and base 12 are sealed together. The leadframe and the semiconductor device are sandwiched in between the sealed cap and base so that the lead frame pins extend outward from the ceramic pieces to form package pins. One such structure is referred to as the ceramic dual inline package or CERDIP which is regarded as hermetically sealed. After the seal is completed the metal pins that extend outwardly from the glass seal are typically chemically cleaned and then coated with either a corrosion resistant metal or a solder that will facilitate using the device on a printed wiring circuit board.
The glass seal is an important part of the device manufacture. Since the semiconductor device must be exposed to the glass sealing temperature, it is desirable to employ low melting glasses. Clearly, the conventional sealing glasses that melt above 1000.degree. C. would not be suitable. Accordingly, the well-known solder glasses that melt at a safe temperature are commonly employed. If desired, a well-known devitrifying solder glass can be employed. Such a glass reverts to a ceramic form during the sealing cycle and subsequent to the seal will have a fusion temperature that is far above the sealing temperature.
Commonly used sealing glasses typically include lead oxide which acts to lower the melting temperature. Other oxides ar included to control thermal expansion, light transmission and other physical properties. Unfortunately, these added oxides commonly result in a glass that is prone to chemical attack. For example, the lead oxide may be reduced upon exposure to a reducing ambient and metallic lead will then be present upon the glass surface. At best, the lead will discolor the glass. A sufficient amount will act to short the leads extending out of the package. Such shorting can be catastrophic. In particular, it has been found that the solutions used to plate the leads contain sufficient reducing chemicals to produce a glass attack that results in lead shorting. In the past, this has greatly limited the choice of glasses that will have a suitable sealing temperature. It has been found that lead shorting is a substantial problem particularly in the production of high lead count packages where the space between leads is small.